The present invention relates to a bipolar transistor and more particularly to a lateral bipolar transistor that are used in bi-CMOS or CMOS integrated circuits.
A lateral bipolar transistor have less steps and lower costs in its manufacturing process than a vertical bipolar transistor that are used in analog circuits and therefore, are used as power circuits and high-frequency amplifying devices in bi-CMOS or CMOS integrated circuits.
The structure of a conventional lateral bipolar transistor is shown in FIG. 1. FIG. 1(A) is a plan view and FIG. 1(B) is a sectional view along a dotted chain line A-Axe2x80x2 shown in FIG. 1(A). Here, a PNP transistor is shown as an example.
On a P-type semiconductor substrate 11, a square device area 13 and a rectangular base contact area 14 are formed which are surrounded by an isolating insulation layer 12 formed at a specified depth from a surface of the semiconductor substrate 11. In the device area 13 and the base contact area 14, an N-type base area 15 is formed on the entire areas. The base area 15 is formed to a depth deeper than the thickness of the insulation layer 12 from the surface of semiconductor substrate 11 and its lower end is turned over to the bottom surface of the insulation layer 12, forming one well area as a whole as shown in FIG. 1(B).
Next, on the central portion of the device area 13, a square P-type emitter area 16 is formed as shown in FIG. 1(A). This emitter area is formed in a depth shallower than the thickness of the insulation layer 12 from the surface of the semiconductor substrate 11 as shown in FIG. 1(B). Accordingly, the P-type emitter area 16 is formed in the N-type base area 15. Further, in the device area 13 around the emitter area 16, a P-type collector area 17 is formed with a specified space from the emitter area 16. The collector area 17 is formed with the same thickness as the emitter area 16 in the N-type base area 15.
On the surface of the semiconductor substrate 11 where the base area 15, the emitter area 16 and the collector area 17 are thus formed, an oxide film 18 is formed for protecting the surface. Contact holes are formed on an oxide film 18 through which base electrodes 19, an emitter electrode 20 and collector electrodes 21 are formed.
The operation of a conventional lateral PNP transistor in this structure will be explained.
Carriers injected from the emitter area 16 into the base area 15 is diffused in the base area 15 and reaches the collector area 17. An emitter-base junction surface is formed by a side surface and a bottom surface of the emitter area 16. The operation of a lateral PNP transistor is primarily contributed by an side area facing the collector area 17. In other words, a current value I injected from the emitter is a sum of a current value IL flowing from the side surface of the emitter area 16 to the collector area 17 and a current value IV that is diffused in the base area 15 from the bottom of the emitter area 16 and flowing into the base electrode 19. However, the current attributing to the operation of the transistor is mainly the lateral component IL since the current value IV becomes a base current by being recombined in the base area 15.
To increase a current amplification factor hFE of a lateral transistor, it is enough to make the bottom area of the emitter area 16 small so as to reduce a current value IV flowing into the base electrode 19 diffusing in the base area 15 from the bottom of the emitter area, which hardly contributes to the operation of the transistor. However, when the bottom area of the emitter area 16 is reduced, a peripheral length as well as the side area of the emitter area 16 is also reduced. Therefore, even the current amplification factor itself can be promoted, an absolute value of the current value IL flowing from the emitter area 16 to the collector area 17 also decreases.
Suppose, for instance, the size of the emitter area 16 is about 3 xcexcm2 and a current amplification factor hFE is 100, only 1xcx9c10 xcexcA of collector current is obtained. Thus, the decrease of an absolute value of collector current of a transistor generates such a problem that sufficient current capacity is not obtained when a transistor is used in a power circuit and thus a power circuit having a sufficient load driving capacity is not obtained.
Further, when a lateral transistor is used as a high frequency amplifying device, cut-off frequency fT is desired to be high. By the way, one of factors deciding cut-off frequency fT of a transistor as a high-frequency amplifying device is the junction parasitic capacity. The bottom portion of the emitter area 16 of a lateral transistor contributes little to the transistor operation and operates to lower the cut-off frequency due to a parasitic capacity between the emitter/base junction. While in a vertical transistor, cut-off frequency of as high as 10xcx9c30 GH has been realized at present, in a lateral transistor, only about 200 MHz at the utmost has been realized.
Thus, the load driving capacity and the cut-off frequency of the lateral transistor are in a trade-off relation. That is, when the emitter area is made large in order to supply with a large current, the cut-off frequency drops because of the increased parasitic capacity. In a simple approximation, when the bottom area of the emitter area is increased to two time, the parasitic capacity of the emitter/base junction becomes four times.
Accordingly, an object of the present invention is to provide a lateral transistor with an improved load driving capacity and a cut-off frequency.
Further, another object of the present invention is to provide a lateral transistor with an improved current amplification factor and cut-off frequency in the state where the sufficient load driving capacity is maintained.
In addition, a further object of the present invention is to achieve the above-mentioned objects with a high degree of integration and a cheap manufacturing cost.
A lateral bipolar transistor according to the present invention comprises a device area formed on a semiconductor substrate being surrounded by an isolating insulation layer , a base area formed in the device area having a specified depth from the surface of the semiconductor substrate, a core insulation layer formed in the base area in a depth from the surface of the semiconductor substrate shallower than the base area, an emitter area formed around the core insulation layer in the depth shallower than the core insulation layer, and a collector area formed in the base area at a specified distance from the emitter area in a depth shallower than the core insulation layer.
Further, in the lateral bipolar transistor according to the present invention, a bottom area of the emitter area is reduced to be less than its side area by providing the core insulation layer.
Further, in the lateral bipolar transistor according to the present invention, the emitter area is provided at a nearly center of the device area spaced from the isolating insulation layer
Further, in the lateral bipolar transistor according to the present invention, on the surface of the semiconductor substrate including the emitter area formed around the core insulation layer, a surface protection film is laminated, and a contact hole is formed on the surface protection film to expose the emitter area formed around the core insulation layer so as to provide an emitter electrode in contact with the emitter area through the contact hole.
Further, in the lateral bipolar transistor of the present invention, the base contact area surrounded by the isolating insulation layer is formed on the surface of the semiconductor substrate beside the device area, and the base area is extending into the base contact area via a lower portion of the isolating insulation layer
Further, in the lateral bipolar transistor of the present invention, the collector area is provided at the peripheral portion of the device area spaced from the emitter area, and a number of collector electrodes are provided on the collector area through the surface protection film.
Further, in the lateral bipolar transistor according to the present invention, a number of base electrodes are provided on the base contact area through the surface protection film.
Further, in the lateral bipolar transistor according to the present invention, the device area, the core insulation layer provided at nearly a center of the device area, the emitter area and the collector area formed around the core insulation layer have nearly rectangular patterns on the surface of the semiconductor substrate.
Further, in the lateral bipolar transistor according to the present invention, the core insulation layer is made of almost the same material as the isolating insulation layer and is formed with a depth being not less than that of the isolating insulation layer from the surface of the semiconductor substrate.
Further, in the lateral bipolar transistor according to the present invention, the base area is a well area having a different conductive type from that of the semiconductor substrate.
Further, in the lateral bipolar transistor according to the present invention, the surface protection film is laminated on the surface of the semiconductor substrate including the emitter area formed around the core insulation layer, and a plurality of contact holes are formed in the surface protection film through which a plurality of emitter electrodes are provided so as to contact with the emitter area.
Further, in the lateral bipolar transistor of the present invention, a plurality of emitter electrodes are provided partially on the core insulation layer or at positions close thereto.
Further, a lateral bipolar transistor according to the present invention comprises a device area formed on a surface of a semiconductor substrate surrounded by a isolating insulation layer, a base area formed in the device area to a specified depth from the surface of the semiconductor substrate, a core insulation layer formed in the base area to a depth from the surface of the semiconductor substrate shallower than the base area with one end being connected to the isolating insulation layer and the other end being extended into the device area, an emitter area formed around the core insulation layer with a depth shallower than the core insulation layer, and a collector area formed in the base area spaced at a specified distance to the emitter area with a depth shallower than the core insulation layer.
Further, in the lateral bipolar transistor according to the present invention, the device area is formed in the U-shape around the core insulation layer.
Further, in the lateral bipolar transistor according to the present invention, a base contact area surrounded by the isolating insulation layer is formed on the surface of the semiconductor substrate beside the device area, and the base area is extended into the base contact area via the lower portion of the isolating insulation layer
Further, in the lateral bipolar transistor of the present invention, the base contact area is formed in an U-shape around the device area.
Further, in the lateral bipolar transistor according to the present invention, a surface protection film is laminated on the surface of the semiconductor substrate including the emitter area and the collector area formed around the core insulation layer, and a plurality of contact holes are formed in the surface protection film on the emitter area and the collector area formed around the core insulation layer. A plurality of emitter electrodes are provided so as to contact with the emitter area through the contact holes, and a number of collector electrodes are provided so as to contact with the collector area through the contact holes.
Further, in the lateral bipolar transistor according to the present invention, the emitter electrodes are partially provided on the core insulation layer or at positions close thereto.
Further, in the lateral bipolar transistor according to the present invention, the collector electrodes are partially provided on the isolating insulation layer or positions close thereto.
Further, in the lateral bipolar transistor according to the present invention, a plurality of base electrodes are provided on the base contact area through the surface protection film.
Further, in the lateral bipolar transistor according to the present invention, the device area is nearly rectangular and the core insulation layer of which one end is extended in the device area is nearly rectangular in the surface of the semiconductor substrate.
Further, in the lateral bipolar transistor according to the present invention, the core insulation layer is made of almost the same material as the isolating insulation layer and is formed in the depth being not less than that of the isolating insulation layer from the surface of the semiconductor substrate.
Further, in the lateral bipolar transistor according to the present invention, the base area is a well area having a differing conductive type from the semiconductor substrate.
A method for manufacturing a lateral bipolar transistor according to the present invention comprises steps of forming a mask pattern on a surface of a semiconductor substrate covering a device area to be formed on the semiconductor substrate, a base contact area and a core insulation layer area to be formed in the device area, etching the surface of the semiconductor substrate through the mask pattern to form recesses thereon, laminating an oxide film on the surface of the semiconductor substrate after removing the mask pattern, exposing the surface of the semiconductor substrate by polishing the oxide film to leave the isolating insulation layer and the core insulation layer in the recesses formed in the etching step, forming a well area having a first conductive type in a specified depth from the surface of the semiconductor substrate at the device area and the base contact area surrounded by the isolating insulation layer, and forming an emitter area and a collector area having a second conductive type in the well area.
Further, in the method for manufacturing a lateral bipolar transistor according to the present invention, the core insulation layer is formed with the same material, quality and thickness as the isolating insulation layer
Further, in the method for manufacturing a lateral bipolar transistor according to the present invention, the emitter area is formed in the well area around the core insulation layer.
Further, in the method for manufacturing a lateral bipolar transistor according to the present invention, the emitter area is formed with a depth shallower than the dept of the core insulation layer.